1. Field of the Invention
The present invention relates to the preparation, and in particular the extrusion, of electrically conducting nonporous thin sheet elements from polyolefins, useful especially in the construction of bipolar plates for electrochemical cells.
2. Prior Art
Many attempts have been made to make conductive or semi-conductive materials from polymeric plastics loaded with conductive solids such as carbon black, graphite, or finely divided metals. All manner of thermosetting and thermoplastic resins have been proposed including melamine, phenol-aldehyde, and more commonly polyolefins such as polyethylene and graft copolymers thereof and polytetrafluoroethylene. See, for example, U.S. Pat. Nos. 3,591,526 and 3,673,121.
In general, compositions of relatively low resistance can be prepared by dry mixing a finely divided thermoplastic polymer and conductive filler and molding the mixture under heat and pressure. Such molded products are normally porous and nonhomogeneous in structure, and accordingly are not suitable for certain sophisticated applications which require thin impermeable conductors of highly uniform composition. An example of such a sophisticated application is a bipolar plate for a fuel cell or battery. In this regard, see, for example, U.S. Pat. Nos. 3,814,631 and 3,530,003.
A far higher degree of homogeneity than is obtained by a dry blending process can be obtained by the use of known mixing devices such as a Banbury mixer or roll mill. French Pat. No. 1,305,140 describes the preparation in a Banbury mixer of a number of blends of carbon black or graphite in a crystalline polypropylene with or without an amorphous copolymer plasticizer. The resistivities of these blends were all of the order of a number of megohms-cm and they are described as suitable for use as thermistors and semi-conductors. In addition, the mixing time in the Banbury mixer is of the order of 30 minutes and this inevitably results in some thermal degradation of the polypropylene with consequent impairment of its physical and mechanical properties. Moreover, such carbon-plastic blends are not capable of being easily formed, for example, by extrusion, into thin nonporous sheet-like structures, particularly thin sheet elements, having thicknesses, for example, of from about 150.mu. to about 500.mu..
When carbon black is incorporated in a polymer such as rubber or polypropylene in a Banbury or similar mixer, there is an upper limit to the amount that can be incorporated to give a homogeneous product. When this limit is exceeded, heterogeneous particles or carbon exist in the mixture and can be identified as such in a number of different ways. The greater the amount of carbon which a given polymer can take up, the better is what can be called its binder efficiency. Three test methods used to measure binder efficiency are briefly described below. In all cases the loaded polymer was ground into granules of about 2 mm diameter.